The invention relates to glass powders that may be employed in cements useful in medicine or dentistry, e.g., in bone or dental cements, especially in the so-called glass ionomer cement.
Glass ionomer cements have been described in German patent application (OS) No. 2,061,513. They consist of a calcium aluminum fluorosilicate glass powder and a mixing fluid which may generally be designated as an aqueous solution of a polycarboxylic acid. The resulting composition may be employed, for example, as a permanent filling material in dentistry. Its significance resides in the fact that it is the first tooth filling material that is satisfactory in cosmetic and mechanical respect and that is simultaneously physiologically so unobjectionable that it may be directly filled into the tooth without any underfilling and similar measures.
A disadvantage of the glass ionomer cements resides in their high water sensitivity during and after the setting reaction. While during the setting reaction the water sensitivity is hardly avoidable for principal reasons, the water resistance of the compositions after setting can be improved in two ways, namely by the use of a highly reactive powder composition or by the use of an especially reactive setting fluid. Especially favorable results are naturally obtained by the combination of the two possibilities. However, in that case the reaction is so rapid that the processing period, i.e., the period of time available for introducing the cement into the cavity and modelling it therein, is extremely short. In many cases hardening takes place already during mixing. In order to ensure sufficiently long processing time, less reactive powders and less reactive fluids are employed, so that the preparations presently available on the market are all water-sensitive for a longer period of time.
From German Pat. No. 1,267,589, it has been known to treat glass grit used in the production of porous sintered structures (filters) with bases or acids in order to cleave the surface of the glass particles.
It is an object of the invention to reduce the water sensitivity of cements useful in medicine and dentistry and based on glass powders and polycarboxylic acids while at the same time ensuring a sufficiently long processing period.
It has now been found that this object is realized when the powder component of the glass ionomer cements is a calcium aluminum fluorosilicate glass whose powder particles are on their surfaces depleted of calcium ions as compared with the average composition thereof.
Therefore, the subject matter of the invention is the calcium aluminum fluorosilicate glass powder described in the patent claims and its use.